Calcium is one of the essential elements for a variety of animals including human, and is intimately involved in the various reactions occurring in vivo. Thus, a measurement of the level of calcium in serum or plasma is one of the important clinical laboratory parameters.
A dry test strip produced by applying to or impregnating in a support a reagent which reacts with calcium to generate color and then drying the support is commonly used in order to rapidly measure a blood calcium level in initial diagnosis such as mass medical examination and screening study.
Unexamined Japanese Patent Application KOKAI Publication No. H05-232108 discloses a dry test strip which comprises a support, a reagent layer formed on part of the support surface, a reagent holding layer which covers at least part of the reagent layer and at least part of the support. The reagent holding layer is formed by braiding synthetic fiber filaments. Thus, a liquid specimen spotted onto the reagent holding layer is absorbed and spread to contact with the reagent layer. If the liquid specimen contains a substance of interest, the substance reacts with the reagent contained in the reagent layer to generate color. A method for determining the presence of color generation or intensity of color visually or using an optical measurement means (colorimetric method) can be used to rapidly and conveniently detect or quantify a substance of interest.
Known colorimetric methods for measuring calcium include the OCPC method in which calcium is allowed to chelate with o-cresolphthalene complexone (OCPC) under alkaline conditions. A dry test strip for measuring calcium which the strip applies such OCPC method uses an alkaline buffer such as a boric acid buffer (pH 9.2).